The Sirius Sentinel is awesome. Way grippier than the Q, but almost just as good as the Millennium plastic. I got one at 174g, and I'm using it as my main overstable mid. But now I have one in all 3 plastics. Seems just as stable as the Q, but that added grip really sold me.
I haven't thrown a Vector, but I throw the Limited Edition Sanny MF in Mill plastic, and the Q-Sentinel. I love the plastic on the ltd edition, which you can find pretty cheap. It's amazingly tacky, if grip is your thing. However, I like the Q more because of it's consistency, and slightly more fade. I haven't thrown a Sirius Sentinel.
As far as throwing them, they require a very good release, as a lot of overstable mids do. I've heard the Vector is the same way. I don't throw FH with them unless it's a serious right dogleg, as the hyzer release has to be pretty dead on. BH is easier to throw with it. Overall, I love the Sentinel. It's been in my bag for years. It's called the Sentinel for a reason, as it's very pinpoint and just so accurate.
I do, however, want to try a Vector. Although I throw both Sentinels pretty consistently, I'd love to trade you for your Vector. If you are strictly looking for a Sentinel though, I might be down to trade one. Let me know!
Hey man, how do you like the Sentinel MF?... considering one currently. Kind of falling out of love with my Vector, and I have a huge crush on Millennium, so I figured I'd give it a shot. Do you have experience with all 3 plastics?